This invention relates to the production of ceramic tapes and is particularly directed to production of silica "green" tapes, especially applicable in the fabrication of ceramic, multilayer circuit boards by a single firing-(or "co-firing") of stacked circuit and dielectric layers comprised of such silica "green" tapes.
A co-fired ceramic multilayer is a high-density electrical interconnection system in a single monolithic structure, fabricated by a single firing of the stacked and registered circuit and dielectric layers. The layers are made from a ceramic medium, typically cast alumina tape containing binders and other constituents, the individual layers having been screen-printed with appropriate conductor materials.
In this process, a ceramic powder, typically alumina, is mixed with a binder system, and the resulting slurry is cast on a suitable carrier, such as a polyester tape using a doctor blade. The dried, unfired cast tape or sheet is called a "green" tape.
The tape is then subjected to a screening process to lay down a circuit pattern on the individual tapes, and such tapes are then stacked and registered, and the laminated layers are then co-fired, e.g., at temperature of 1600.degree.-1700.degree. C. for several hours in a high-temperature furnace, to form a multilayer board.
Alumina tapes processed by co-fired methods yield lower frequency electronic packages. However, multilayer circuit boards made from relatively high dielectric constant materials, i.e., alumina tapes, do not operate efficiently at high frequencies required by GaAs circuits. Further, such dielectric materials used in a co-fired process operated at the above high temperatures limit the use of the electrical conductors of the circuit patterns to those of the high-temperature refractory metals, such as tungsten and molybdenum, rather than the higher conductivity metals copper and gold, which have lower melting points below the above-noted high co-firing temperature. Additionally, the final firing of the alumina substrates produces dimensional changes in the X and Y directions which are not uniform.
Use of other dielectric tapes, e.g., organics such as teflons and polyimides, are inherently non-hermetic or are difficult to process effectively.
Silica tapes, if capable of being processed effectively by co-fired techniques, would yield electronic hermetic packages with electrical properties conducive to high frequency operation. However, commercially available silica powder, such as fumed silica, because of its extremely high surface area, cannot be processed into "green" ceramic tape without cracking during drying.
The production of ceramic tapes employing alumina or beryllia, and their use in the production of multilayer boards or integrated circuit packaging, as by co-firing layers of such tapes containing individual circuit patterns, is disclosed in the articles "New Ceramics Fill Performance Gaps" Ronald Pound, Electronic Packaging and Production, September, 1987, pp. 30 and 31; "Co-Fired Ceramic Multilayer; When Reliability Counts" Robert Keeler, Electronic Packaging and Production, May, 1987, pp. 40-42; and "Making Thin, Flat Ceramics--A Review" E. P. Hyatt, Ceramic Bulletin, Volume 65, No. 4 (1986).
It is an object of the present invention to provide improved ceramic "green" tape, adapted to be screen-printed with various conductors, laminated to form multilayered structures and co-fired at substantially reduced temperatures as compared to alumina tapes, to produce multilayer circuit boards or interconnects for high frequency applications.
Another object is the provision of silica "green" tapes, which do not crack during firing and which can be screen-printed to form a circuit pattern, and a plurality of such tapes arranged in registered, stacked form and co-fired to produce improved multilayer circuit boards having low dielectric constants and which operate efficiently at high frequencies.
Yet another object is to provide procedure for production of the aforementioned ceramic "green" tapes and for co-firing such tapes containing circuit patterns-in a registered multilayer arrangement, for production of improved multilayer interconnects or multilayer circuit boards.